Almost since the early days of cable and wireless communications,
it became quite clear that the higher the carrier frequency, the wider
the bandwidth and hence the greater the volume of messages that could be
carried. Today, the utilization of optical energy in communications
provides frequencies in the range of [10.sup.13] to [10.sup.14] Hertz
which allows engineers to iron out a vast number of transmission
capacity problems.

The properties of light as an information carrier had already been
researched some 50 years ago. However, there were two problems standing
in the way of its practical application. The first was the lack of a
light source which could be easily modulated and emit light on a single
frequency. The second was the trouble-free propagation of the modulated
signals. The projection of a light beam through the air was subjected to
atmospheric conditions, under which absorption, scattering, diffraction,
etc. - commonly called attenuation - occur in unpredictable ways. This
meant that an optical communications system had to be powerful enough to
overcome natural attenuation, otherwise it would be useful only under
very benign atmospheric conditions. It thus became necessary to find a
medium - a waveguide - which would produce a minimum of attenuation and
also "guide" the waves around bends and corners. This had to
be cheap and easy to install and maintain. The first requirement was to
develop a powerful yet small light source which could be modulated. The
breakthrough came in 1960 with the invention of the laser at the Hughes
Laboratories, and the development of light-emitting diodes (LED), both
of which can generate well collimated light beams of high intensity.
Both devices can be simply controlled and modulated with low-voltage
signals. This placed the dream of communicators within reach - an almost
unlimited volume of simultaneously carried light signals.

Birth of the Fiber Optic Cable

Two approaches were initially pursued for the development of a
practical waveguide. The first was the use of gas-filled tubes in which
temperature was controlled at selected points for steering the
refractive index of the gas. The second was the use of glass/quartz
fibers in which the required refractive index was varied across the
radius of the fiber during manufacture. This second approach achieved
the desired goal in the late sixties when a light attenuation factor of
only 20dB/km could be demonstrated.

The modern optical fiber cable was born. It has since seen a rapid
progress. By 1980 attenuation had been lowered to less than 1 dB/km and
modulation rates of several Giga-Hertz (GHz) became possible. In the
mid-eighties, when fibers with losses of 0.5 dB/km could already be
fabricated, the light intensity was approximately halved every six
kilometers. This low attenuation loss was achieved at the relatively
long wave-length of approximately 1.3 meters, located in the infrared
region. Marconi has recently demonstrated an optical fiber system
offering 0.2 dB/km attenuation with a modulation rate of up to 20 GHz.
By way of illustration, this bandwidth is capable of carrying about six
million telephone conversations or 3 000 TV programs simultaneously in a
mono-mode glass fiber of only one-eighth of a millimeter in diameter.
The very latest fiber cables operate on wavelengths in the 1.55 meters
range, which has by now become a universally accepted standard, and with
0.18 dB/km come very close to the theoretical attenuation limit of 0.14
dB/km.

Lasers vs. LEDs

Compact and powerful semiconductor diode laser chips were being
developed in parallel. The devices have active layers of gallium indium
arsenide phosphide grown on indium phosphide substrates. However lasers,
in particular those for the longer wavelengths, are difficult to
manufacture, are expensive and require special precautions when used in
optical fiber transmitters. For these reasons there was at first a great
interest in LED transmitters, which are based on the same materials as
the lasers but have simpler structures and are easier to handle in
optical fiber systems. But there is a penalty: the LEDs' optical
power is lower and they have a much wider light spectrum. Which type,
LED or laser, is finally used depends on the system's requirements.
The laser appears now to be emerging as the winner because types have
been developed lately which emit on the single precise wavelength of
1.55 meters, exactly where the lowest attenuation rate in the light
spectrum is found. Considerable pioneering work in this field has been
performed by firms like Plessey, Hughes, Siemens, SEL, Thomson and
numerous other laser specialists.

The Role of the Repeater

An essential part of any optical fiber system designed for
long-range communications is the repeater (also called pulse restorer or
electro-optic regenerator). Light signals can be transmitted through an
optical fiber either digitally, by pulsing the light, or in analog form,
by varying its brightness. The usable bandwidth results from the maximum
rate at which the pulses comprising the signals can be sent and received
without error. A form of attenuation called dispersion sets a practical
limit to that rate. The typical speed of a digital on/off pulse is one
billionth of a second; here dispersion means that a pulse is spread out
in time and part of it arrives too late to be interpreted correctly at
the receiving end. To "boost" the pulse one uses
semiconductors incorporating a receiving system (usually an avalanche
photo diode), amplifiers and diode lasers or LEDs as transmitters. This
assembly, usually mounted on chips, receives and reforms the light beams
and re-injects them further down the optical fiber. The power required
for the operation of the repeater - a mere 1/1000th of a Watt -, is
almost negligible. Technically, the repeater's design is not unlike
that of the transmitter and receiver at the terminal ends of the
communication line, where the digital signals are demultiplexed,
amplified and forwarded digitally, or demodulated to analog format.

Advantages of Optical Fibers

The primary impetus for the development of optical fibers
originated in the commercial and not the military field. The
communications industry was beset with two major problems; first the
laying and maintenance of costly conventional undersea cables, whose
capacity by the late sixties was becoming insufficient; and the
high-risk ventures of operating satellite networks, which have an
average life expectancy of only seven years. It had therefore been
looking for a long time for a suitable way of extending these
established means of communication.

ATT-Bell, Japan's NTT, France Telecom, British Telecom, German
Telecom, Northern Telecom, plus communications equipment producers such
as CIT Alcatel, Philips, AEG Kabel, Siemens, GTE, Plessey, Marconi,
Italtel. RCA, Siecor (a joint venture between Siemens and Corning Glass
in the USA) or Raytheon, just to name a few, have over the past 30 years
invested billions in optical fiber research. Today optical fiber
undersea cables, no thicker than a garden hose but capable of carrying
up to 80 000 signals simultaneously, link Europe, America and Asia.
German Telecom has already close to 800 000 km of cables in operation.
This is only the beginning, since fiber optics, next to microelectronics
and software, is regarded today as one of the three key technologies of
the future. It is surprising that the military and the defense industry
embarked rather late on utilizing optical fibers in spite of their
obvious military potential, as the following amply demonstrates.

Apart from their large message-carrying capacity the greatest value
of optical fiber systems is their very high data transmission rates:
today these reach several hundred megabits per second (Mb/sec). This
permits the design of networks with an inherent multiple redundancy in
cabling which serves to increase overall survivability. The electric
power needed to run the system is minimal because the optical elements,
such as the laser or LED diodes, need virtually no amperage.

Of even greater attraction to the military is that optical fiber
systems are immune from all types of electro-magnetic interference. They
are thus the most secure means of communication since intercept is
virtually impossible. In the first place the sheer mass of data passing
through the cable will make it extremely difficult to extract a specific
signal. Secondly, the lines cannot be tapped without alerting the users.
In addition glass cables are ECM-and EMP-proof; they feature complete
electric isolation, as no part of the system has to be grounded; and
they generate no electric field and no electrical hazards. If the lines
are well camouflaged they are difficult to detect because they need not
be much thicker than a pencil, even for mass communications, and for
simple temporary communication tasks or the guidance of weapons of
ground-, sea- and air-launched weapons they can be hair-thin. Even heavy
duty optical fiber cables weigh up to 10-20 times less than a copper
cable of equivalent capacity. A further advantage in their manufacture
is that in contrast to copper, which is in short supply, cheap silica
and quartz needed for glass production are available in abundance. A
properly designed and manufactured cable is rugged and durable, it will
withstand temperature extremes and features an inherent material
resistance to damage far superior to that of any multi-wire, coaxial
copper cable. Standard optical fiber cables produced for the
communications industry withstand proof stresses of 3 500 kg/[cm.sup.2],
those made for mobile military purposes almost ten times as much. A tank
can roll over a fiber cable without causing damage.

The Cost Factor and Other Draw-backs

Optical fiber-based systems do have disadvantages, however. They
are still about three times as expensive as conventional systems.
Although the cable as such costs considerably less than its copper
equivalent, it is the complexity of the laser and/or LED diodes which
must be incorporated in special highly integrated chips that raises the
cost. In addition, customized interfaces to link old and new
communication systems have to be developed, and new man-machine
interfaces have to be designed.

A German Army policy paper issued in 1985 stated that although
implementation of fiber optics was desirable, it was impractical for
mobile warfare where radio and data link traffic was preferable. For
stationary peacetime use the networks of the public telecom services
were to be utilized. This may be a reason why in case of a conflict NATO is thinking of tying special optical fiber equipment modules into the
Telecom-owned ISDN (Integrated Services Digital Network) optical
fiber-based networks of the future, instead of setting up duplicate
tactical or strategic [C.sup.3]I optical fiber networks. However, this
attitude may change when the old generation of [C.sup.3]I systems is due
for replacement by the turn of the next century.

A compromise solution, not unlike that adopted by the
communications industry, is the gradual introduction of optical fiber
systems at those points in existing military systems faced with signal
traffic density and other critical operational problems. In the military
such problem areas are the larger headquarters and some important
[C.sup.3]I network nodes. For meeting the requirements of the German
Army, SEL Alcatel has developed its TACLAN (TACtical Local Area
Network), which is currently being put into service. It provides
integrated communications services for brigade, division and corps
headquarters.

TACLAN and ATICOS

Designed for operation on the purely local headquarters level,
TACLAN consists essentially of two common basic component types -
field-deployable fiber cables and so-called network access units (NAU).
The cables and NAUs constitute a distributed exchange, each access unit
providing access to the system for eight voice connections and four
digital accesses for computers, data banks, work stations and other
[C.sup.3]I elements. Voice and data traffic is handled simultaneously as
the optical fiber bus transmission rate of 10 Mb/sec is easily capable
of handling the requirements of up to 60 subscribers. If needed, the
network access unit's operation and accessibility can be regulated
by a commercial micro-computer. The optical fiber cabling serves to link
the various vehicle-mounted shelters of the command post, which can thus
be dispersed over a large area for better camouflage and protection.

Another typical product for meeting the almost identical
requirement is the Swiss Siemens-Albis ATICOS (Albis Tactical Integrated
Communication System). Essentially the equipment is a flexible, mobile,
militarized version of a commercial ISDN node. It can thus provide fast,
secure and reliable communications for voice, data, fax and video and
can interface with new optical fiber systems or existing conventional
networks. All the required options to interconnect with fiber, copper
cable or radio nets are included as standard features. ATICOS' user
ports can be configured by the software to support either analog or
digital terminal equipment of a large variety of interface standards.
This means that virtually any existing communications network, be it
commercial, private or military, can be accessed and utilized.

FOTS (LH)

Both the above systems have been designed to operate with
conventional long-range communications such as data-link or radio. For
the US Army's Fiber Optic Transmission System (Long Haul) or FOTS
(LH) program, ITT's Defense Communication Division is developing
equipment suitable for mobile field use. The equipment available
hitherto meets the goals of mobility, easy deployment and survivability
required for tactical cabled communications networks. The target of the
program is the eventual replacement of all twin, metallic, coaxial
[C.sup.3]I cables. Apart from its primary long haul function the new
system will provide radio remoting and inter-shelter connections at
headquarters. In the long haul application the system transmits optical
signals over distances of up to 64 km at the rate of 20 Mb/sec. Small
battery-powered repeaters are needed at every six kilometers. The 5.5
mm-diameter cable, named in military terminology FOCA (Fiber Optic Cable
Assembly), is available in standard lengths of one kilometer weighing 29
kg, and is fitted with standard connectors for easy coupling. These
connectors are highly critical elements if numerous FOCAs are joined for
long haul transmissions. A connector interrupts the smooth flow of light
waves in the cable's optical core. The task was to produce a sturdy
connector joining two cable ends so tightly that only minimal losses
occur. This means that each connector must contain optically polished
and physically protected cable ends. Although such connectors are
high-precision optical instruments they must be able to withstand rough
use in military service. STC Defence Systems of the United Kingdom is a
specialist in this field and collaborates with ITT, GTE, Plessey,
Siemens and the research establishments of the British and US armies.

Connectors

The STC-produced FOCA connectors and cables can be immersed in
water, are resistant to sand, dust, salt spray, fuels, fungi and oils
and can be mated 2 500 times before showing any loss of performance. The
cable as such will withstand 100 impacts of 1.5 kg and can be bent
sharply at the same place more than 2 000 times before suffering any
attenuation. In terms of normal use the FOCA is pretty well
indestructible. The signal attenuation of the standard one-kilometer
cable and its two connectors is about four decibel.

The implementation of such FOTS (LH) systems for the US Army is
going ahead. GTE has already received the contract for providing 18 TGCR (Tactical Generic Cable Replacement) terminals. Each terminal will
replace a 26-pair copper cable, leading to major reductions in weight
and volume in addition to high security and almost total ECM and
EMP-protection. GTE claims that TGCR, for which Plessey is under
contract to provide the LED diodes, will under identical conditions
carry signals further than copper cable systems.

Long Haul Cable Networks

Apart from serving command and control functions, such long haul
cable networks will eventually offer surprising benefits in the
real-time combat intelligence field. Each terminal of an optical
fiber-based [C.sup.3] network offers direct access to any subscriber,
who can rapidly and directly insert an enormous quantity of information
without overloading the system. It is therefore possible to feed into
the network seemingly irrelevant information which can be filtered and
evaluated by computers, operated with neutral/artificial intelligence
software (see Armada 1/90), for previously established target
parameters.

A potential source for such data might be optical fiber-guided,
anti-tank and air defense missiles currently under development. The best
known missiles are the Boeing/Hughes FOG(M) or Euromissile's
Polyphemus projects. The MBB Division of Deutsche Aerospace is actively
engaged in this field and six years ago flew experimental anti-tank
missiles equipped with TV and IR cameras in the nose. MBB uses a very
thin lightweight optical guidance fiber more than 15 km long with an
optical attenuation of only 1.4 dB/km. During the missile's flight
the camera can scan the battlefield, and the images can be fed
simultaneously into the [C.sup.3]I net for instant evaluation. For
battlefield surveillance drones can be tethered by a lightweight optical
fiber to a forward ground station which can feed the recce results
directly into the [C.sup.3]I net. The image resolution attained is of
photographic quality. This image quality, however, is not dependent on
the overall performance of the transmission medium - as is the case with
microwave data-links - but depends exclusively on the resolution of the
camera's optics.

Weapons Systems Applications

The options for the use of optical fiber are virtually unlimited.
Cameras might also be mounted on tank or special, unmanned robot
vehicles tethered by a hair-thin 20 to 30 km-long cable providing the
lower echelon commanders with a first-hand view of the action. Such air
and ground-based reconnaissance systems are currently under study. Tied
into a FOTS (LH) type net, they can give the theater commander for the
first time in history real-time recce data.

The options go far beyond that. Combat robots can be controlled
from secure locations and remote operation of conventional weapons is
possible because the information flow which can be carried by the almost
totally ECM-proof fiber links is virtually unlimited.

The reconnaissance possibilities offered by optical fiber
technology are also of great interest to naval forces. An IR or
TV-camera system lifted to 8-10 km altitude by a rocket can enormously
extend the visible horizon. The images can be video-taped and replayed
in slow motion for thorough evaluation.

Such rocket-carried recce systems are of major interest to
submarines since even under the very best sea conditions, the visual
range of a periscope is very limited. The reconnaissance package can
even be launched from submerged submarines.

A similar launcher could be loaded with optically-guided
anti-helicopter, missiles and fired with deadly accuracy from submerged
submarines at attacking ASW rotorcraft. Euromissile, utilizing
MBB-developed optical fiber technology, is said to be working on such a
missile for a new generation of attack submarines.

Torpedos

Thomson-CSF is experimenting with optical fiber-guided torpedos.
Wire-guided torpedos have always suffered from the low message-carrying
capacity and weight of the wire link. The capabilities of the
weapon's sophisticated sonar head could be far better utilized if
an improved control by a high capacity, two-way data link was available.
In addition, the optical fiber would be lighter and less prone to break
than the currently used metallic wire. Although modern torpedos are
equipped to operate independently after loss of direct control, their
efficiency is considerably reduced (they have problems in distinguishing
between a decoy and the target). Optical fiber guidance reduces this
danger and thereby increases the lethality of the weapon.

Conclusion

Numerous weapons will benefit from the large message-carrying
capacity of fiber optics. For example, rockets or guided missiles could
be used for laying small capacity cables from point A to point B over
distances of more than 20 km. These cables, of not more than one
millimeter in diameter, would be fired without connectors.
Simple-to-operate equipment for polishing and splicing fiber cables
already exists and could be used to prepare the bare cable ends for
direct connection with the terminal's interface. A forward
observer, a patrol or an advance group of armored vehicles could thus
tie itself quickly into a [C.sup.3]I network without running the danger
of discovery or radio intercept by the enemy.

GTE has developed and successfully tested a fiber optic cable for
such purposes. It operates with a minimal attenuation and does not
require repeaters. The reformation of the time-distorted signals and
their amplification are performed by an optical re-shaping of the pulses
within the cable. Provided a simple production process can be developed
for the fiber, which consists of a specially doped glass mixture, this
marks an important breakthrough. On the whole, for the military - as
well as for commercial operators - optical fiber technology is about to
open up a new age of perfect, secure and comprehensive communications.

PHOTO : Consisting of seven 225-fiber strands, this Siemens optical
fiber cable can carry up to

PHOTO : 60 000 telephone calls.

PHOTO : Microscope photo of an SEL mono-mode fiber optic cable end.
The fiber is visible in the